Water can be used for different purposes in the process of recovery of hydrocarbon products from a subsea hydrocarbon deposit. One principal use of water in subsea hydrocarbon production is the pumping of water down into water injection wells that are formed in a subterranean hydrocarbon reservoir. Injection of water is a commonly used method for increasing the degree of recovery of crude oil from a subsea deposit. If injection water is taken from seawater the raw water needs to be removed from constituents which may otherwise harm the process equipment or lead to souring and/or fouling of the reservoir ground structure. Production of injection water from seawater therefore typically involves removal of solid particles suspended in the water, removal of dissolved inorganic material such as salts and sulphates or inhibition of organic material/organisms such as bacteria, etc. The process conventionally includes filtration through semipermeable filter membranes, and may additionally include the supply of chemicals such as chlorine or biocide and chemical corrosion or scaling inhibitors, e.g. to the treated water.
In this context two types of filters are usually employed—the dead end filter type and the cross-flow filter type.
In a dead end filter the supplied fluid, i.e. water in this case, is forced towards an upstream face of a semi-permeable membrane. The water that passes the membrane, named as permeate, is removed from organic and inorganic matter which is retained on the upstream side of the membrane, the retained matter named as retentate. A dead end filter generally comprises a membrane in the form of a metal or synthetic wire mesh having a suitable mesh size and filter depth. A filter cake of retentate matter is successively formed on the upstream side of the membrane, eventually covering the openings or channels through the membrane which then calls for replacement. The dead-end filter is relatively non-complex in structure and therefor comparatively cheap. A frequent replacement of filters is thus acceptable, at least in topside applications where filters are readily accessible for easy replacement.
Subsea applications often require equipment with a longer service life. In this aspect the cross-flow filter type can be designed with a comparatively large filter area in relation to its overall dimensions and may therefore be better suited for subsea use. The cross-flow filter designs include spiral wound membranes, tubular or straw membranes and hollow fibre membranes, e.g.
In the cross-flow filter the supplied water is conducted along an upstream side of a semi-permeable membrane. In result of the pressure applied, water is forced through the membrane to a downstream side from where the water is discharged as a permeate flow, relieved of organic and inorganic matter which is retained on the upstream side of the membrane. The retained matter is conducted away from the upstream side of the membrane and is discharged as a retentate flow.
Cross-flow filtration is often used for removal of smaller or very small components in the supplied water, including removal of dissolved components such as salts and sulphates.
In subsea applications and seawater treatment technology filters are generally applied in connection with coarse filtration, microfiltration, ultrafiltration, nano filtration and reverse osmosis filtration. Each of these grades of filtration successively removes smaller components from the supplied water from a particle size in the order of microns with reference to the micro- or ultrafiltration membranes down to particle sizes at ionic level with reference to the nano-filtration or reverse osmosis membranes.
In this connection it serves to be pointed out that an ultra filtration unit can be realized in the form of a tubular membrane in which water is forced through the fiber walls and the matter/solids are retained in one side. In nano filtration units and reverse osmosis units water flows parallel to membrane walls. Due to the high pressure, part of the flow permeates through the membrane walls to get the desired quality water while still a significant amount of reject that did not go through the membrane (richer in ionic content) flows out as a reject stream.
In the production of injection water from seawater, inhibition of bacterial growth may involve the generation of chlorine by electro-chlorination. In electro-chlorination energy is supplied to sodium chloride (NaCl) that is dissolved in seawater which passes through an electric field, resulting in sodium hypochlorite (NaOCl) and hydrogen gas (H2). The sodium hypochlorite is a good alternative for seawater disinfection because its production can be made subsea using an electro-chlorinator.
Electro-chlorination of raw seawater may however also cause problems in terms of scaling caused by precipitation of calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2), forming crystals in the water which may sediment in the system and clog downstream filtration units and membranes.